The biomechanical mechanisms underlying spinal flexion and extension are unique in comparison with other appendages. The spine is primarily loaded in compression, the magnitude of which can be measured invasively via the internal pressure developed in the nucleus pulposus of the intervertebral discs. The nucleus pulposus at the core of each disc and the surrounding annulus fibrosis together act as a hydraulic cushion that provides uniform hydrostatic force to separate each vertebrae of the spine. The thickness and pressure within each disc are largely dependent on the amount of fluid contained in the intervertebral disc, which can decrease by as much as 20% over the course of a day of normal activity in healthy people. The magnitude of the pressure developed in the intervertebral disc is directly related to the degree of postural back muscle activation.
Spinal orthoses are often prescribed to lessen the degree of postural back muscle activation by restricting the motion of the spine and/or offloading the spinal column following spinal surgery or trauma. Spinal offloading is typically achieved through one of two mechanisms. First, the application of three-point bending to the trunk in the sagittal plane can reduce spinal loading. An example is the Jewett hyperextension orthosis (Florida Brace Corp., Orlando, Fla.), which applies two posteriorly-directed forces at the thorax and pelvis and one anteriorly directed force at the lumbodorsal region of the trunk, offloading the anterior thoracolumbar vertebral bodies to promote the healing of compression fractures. Although this type of orthosis allows for a range of motion, little or no support is provided until the wearer reaches the limits of the range of motion, where the structure of the orthosis comes into sustained contact with the wearer and support is applied.
Second, increasing intraabdominal cavity pressure can reduce spinal loading. Lumbosacral orthoses, such as soft belts and semi-rigid corsets, can reduce the muscle effort required to maintain a stable neutral posture. Soft belts and semi-rigid corsets are the most commonly used forms of spinal orthoses used outside of a clinical setting, and they are typically used with the intention of preventing or treating lower back pain of workers performing occupations with frequent bending and lifting. Despite the common use of such orthoses, the use of such commercially-available back belts does not provide a reduction in the likelihood of injury. Custom-made orthoses produced by a trained orthotist have been shown to be more biomechanically effective than common off-the-shelf models, but have several predominant drawbacks. First, the individual manufacturing and fitting required are prohibitively expensive for common usage. Second, the restricted maneuverability such orthoses create could be disadvantageous in the workplace. Lastly, some custom-made corset-style orthoses can result in increased back postural muscle activity that could promote muscle fatigue.
Spinal orthoses in general reduce the effort required of the postural muscles and also the compressive load they add to the spine. This is accomplished when an orthosis produces its own corrective moment, created from normal contact stress applied to the skin of the user. However, as mentioned above, current orthosis designs that allow for a range of motion provide little or no support until the wearer reaches the limits of the range of motion. At this point, the structure of the orthosis comes into sustained contact with the wearer and support is applied. Thus, there is a gap in support when maneuverability is allowed. For the foregoing reasons, there is a need for a non-custom orthosis which would reduce spinal compression and muscle effort in resisting gravitational bending moments due to the mass of the trunk, while simultaneously allowing multi-planar maneuverability within a set range of motion.